In recent years, various difficultly degradable harmful chemical materials have been detected in soil, rivers, sea, air, etc. The progress of contamination by these materials has caused problems. In particular, the contamination of soil by heavy metals or organic chlorine-based compounds has caused serious problems. It has thus been keenly desired to prevent the expansion of contamination as well as establish a technique of remedying polluted environment. For example, waste disposal site of chemical plants handling heavy metal compounds, gas production plants, oil refineries, fuel bases, pulp plants, etc. or site at which these plants used to be located are facing the problem of soil contamination. Thus, there is a growing demand for improvement and development of soil remedying method for purifying the contaminated soil.
Further, since contaminated soil not only prevents the reuse of land but also is much likely to allow the contaminants to flow and diffuse into underground water, causing the expansion of polluted area, it has been keenly desired to establish as soon as possible a technique for remedying contaminated soil.
As soil remediation methods for removing contaminants from contaminated soil to restore the soil to original state there have been known and attempted various methods.
For example, there are known physicochemical methods such as vacuum extraction method involving the suction of contaminants from soil. However, these physicochemical methods are disadvantageous in that they take much cost, have a lowered working efficiency and can difficultly dispose contaminants present in a low concentration.
Under these situations, among soil remediation methods, remediation by soil purification utilizing microorganisms, i.e., so-called bioremediation has been much expected of. As soil remediation methods utilizing microorganisms there have been attempted a method involving the enhancement of the activity of microorganisms originally occurring in soil to be treated and hence the autopurifying capacity of ecological system of degrading contaminants and rendering them harmless and a method involving an improvement of this technique, i.e., positive external introduction into soil of bacteria capable of degrading contaminants (hereinafter occasionally referred to as “contaminant-degrading bacteria” or simply “degrading bacteria”) leading to the acceleration of remediation of contaminated soil.
Numerous difficultly degradable compounds causing soil contamination, e.g., microorganisms which degrade aromatic hydrocarbons and organic chlorine-based compounds have been known. However, if these bacteria are merely sprayed onto actual contaminated soil, the concentration of bacteria in the soil decreases from the initial vale with time, lowering the efficiency of remedy in the contaminated site. In order to prevent the drop of remediation efficiency, it is necessary that spray of microorganisms be often repeated, adding to labor and cost. Accordingly, it is practical to maintain the proliferating capacity of bacteria applied to soil and hence keep the concentration of bacteria high.
Thus, it has heretofore been proposed to apply to bioreactors aimed at remedying soil various microorganism carriers for retaining microorganisms therein in a high concentration. As such supporting carriers there are known particular carrier materials such as porous glass, ceramics, metal oxide, activated charcoal, zeolite and anthracite and gel-like carrier materials such as agar, polyvinyl alcohol, alginic acid, polyacrylamide and carrageenan.
For example, International Patent Publication No. 1992-503528 discloses a method involving the addition to contaminated soil chlorophenol-degrading microorganisms fixed in a porous carrier. This porous carrier forms an environment desirable for the growth of microorganisms, making it possible to protect microorganisms against the attack by predators.
However, soil is normally oligotrophic, making it occasionally necessary to provide the soil with materials which act as nutrients for soil remedying microorganisms (hereinafter referred to as “nutrients”). In this case, nevertheless, materials move in soil very slowly. Thus, even if nutrients for soil remedying microorganisms are given to soil from the surface thereof, there often occur nonuniformity or variation of nutrient concentration, making it difficult to exert desired effects. Accordingly, even if microorganisms are sprayed fixed on any carrier such as organic, inorganic, particulate, gel-like and porous carrier described above, not to mention if microorganisms are merely sprayed onto soil, the sufficient supply of nutrients to all microorganisms applied can be difficultly accomplished (unless applied excessively), leaving something to be desired in the maintenance of activation of proliferating capacity of applied microorganisms and high bacterium concentration in soil.
In order to overcome these difficulties, Japanese Patent Laid-Open No. 1994-212155 discloses a method involving the filling of pores in a porous carrier for supporting microorganisms with nutrients as an embodiment allowing nutrients to be supplied in such an arrangement that they can be sufficiently used by microorganisms. Further, Japanese Patent Laid-Open No. 1995-97573 discloses a method which comprises injecting into soil a liquid composition capable of forming a carrier for supporting microorganisms and containing nutrients to form a microorganism carrier containing nutrients that can be expected to make a uniform progress of soil purification. Moreover, Japanese Patent Laid-Open No. 1996-1181 discloses a method involving the incorporation of nutrients such as yeast extract in a polymer gel carrier with microorganisms. Japanese Patent Laid-Open No. 2001-501645 discloses a method involving the encapsulation of microorganisms and nutrients therefor in a water-absorbing polymer.
Although these related techniques apparently have some improvements in remediation of contaminated soil, the resulting effects are not sufficient. The reason for this insufficiency can be thought as follows. Even if the form of supply of nutrients is improved as mentioned above, the microorganisms depend on the nutrients applied so long as nutrients are supplied. Thus, the effectiveness of progress of soil improvement with contaminants in soil as nutrient sources is lowered. Further, microorganisms originally occurring in soil consume the nutrients thus supplied, causing remarkable consumption of nutrients and change of nutrient concentration. Moreover, microorganisms originally occurring in soil can inhibit the growth of microorganisms capable of remedying soil. In other words, the supply of nutrients is required for the maintenance of activity of microorganisms capable of remedying soil but comes up against a dilemma that it also lessens the effect of improving soil.